Demands have arisen for a flash memory with a long service life and long-term operational stability. A sense amplifier which detects data held by a memory cell transistor compares a current flowing in a bit line with a current flowing in a reference signal line and determines, based on the potential determined by the comparison result, whether the data held by the memory cell transistor is binary 1 or binary 0.
If, however, a voltage is applied for a long time to the gate of a MOS transistor forming a sense amplifier, which makes a current flow corresponding to the magnitude of a current flowing in the bit line or the reference signal line, the transistor characteristics deteriorate, resulting in a failure to read accurate data.